This invention relates to an RFID label in accordance with the first part of claim 1. The abbreviation xe2x80x9cRFIDxe2x80x9d stands for the English expression xe2x80x9cRadio Frequency Identificationxe2x80x9d.
RFID labels comprise, like the ones considered here, an electrical resonant circuit with a coil and a capacitor generally on a thin plastic substrate as well as a very small integrated semiconductor circuit (chip) which gives the label a certain intelligence. The coil also acts as an antenna so that the label can communicate by xe2x80x9cradioxe2x80x9d as it were with a remote transmitting and receiving station on the resonant frequency determined by its resonant circuit in the xe2x80x9cradio wavexe2x80x9d range. The label draws the energy needed to operate the semiconductor circuit from the external field received via the coil antenna. The label itself therefore has no energy source of its own.
RFID labels can be used in particular, due to their xe2x80x9cintelligencexe2x80x9d, for the identification of articles, at the entrance to a building complex to provide secure access, or also, for example, as a ski-lift ticket.
As a rule, RFID labels are subject, along with their pertinent transmitting and receiving stations, to strict technical regulations which, for example, allow only very strictly limited frequency bands for their operation. Such a frequency band often chosen for RFID labels is, for example, 13,560 MHz, with the width of this band being only xc2x17 KHz.
With respect to RFID labels of the design described, there is now the problem of the precise regulation and maintenance of the resonant frequency of the labels in such a narrow range, especially in the case of large-scale production.
Such a device is known from the prior art in various configurations.
Different methods for xe2x80x9ctrimmingxe2x80x9d the resonant frequency have already been disclosed for secondary correction of a resonant frequency that is not in the desired frequency range.
Such a known trimming procedure is, for example, the cutting down of a capacitor by means of a laser (see U.S. Pat. No. 3,947,934) or a blanking or cutting tool (see WO 89/05984). This makes the area of the capacitor smaller, which reduces its capacitance. The resonant frequency of the resonant circuit is affected by the reduction in capacitance. i.e. the resonant frequency is increased.
Another known technique is the melting of the plastic film between the capacitor surfaces. This reduces the distance between the plates of the capacitor, which causes the capacitance of the capacitor to be increased and thus causes a reduction in the resonant frequency of the resonant circuit.
Both of these known techniques have various disadvantages. For example, they require the use of expensive precision machines which make the production process for such labels much more expensive. There is also the danger of damaging the surfaces when the capacitor plates are cut to size on the film. Additionally, the known techniques cannot be readily implemented at the required speed during the mass production of labels using the continuous operation process.
It is therefore the object of this invention to indicate how the resonant frequency of RFID labels can also be regulated so that they are then in a given frequency band. This also has to be mechanically simpler and therefore more economical than with the known techniques.
This object is achieved by providing the label with an additional element subsequently applied to its surface which changes the resonant frequency of the resonant circuit and brings it into the desired range.
The advantages of this invention can be seen especially in the fact that a sticker with a conductive layer or a layer made of a material with a high dielectric constant can be used as the additional element, the area and/or material of which cause the desired correction of the resonant frequency and which can be very simply and precisely applied, even in large-scale production, and at a high production rate to the otherwise already finished label.
Also, in the method of this invention, the extent of the frequency change caused by the additional element can be chosen to be absolutely larger than with the known trimming procedure mentioned above, which increases the precision of the frequency regulation that can be achieved.
According to a preferred embodiment, a sticker with a conductive layer and an adhesive layer is applied for the purpose of increasing the effective capacitance of the resonant circuit and overlaps, at least partially, at least two conductive surfaces with different potentials on the same side of the substrate. It can be applied on the same side of the substrate as these conductive surfaces, directly on them as it were, with, for example, only the adhesive layer acting both as an insulating and a dielectric layer. On the other hand, however, it can also be applied on the opposite side of the substrate, with the substrate additionally acting both as an insulator and a dielectric. The sticker is particularly effective if the areas selected as the conductive surfaces to which it is applied are at least approximately at the potential of the two plates of the capacitor. Normally these are those areas that are also used for bonding the semiconductor circuit.
Basically, a sticker could also be conductively connected with a conductive surface on one side of the substrate and also be arranged in such a way that it at least partly overlaps another conductive surface on the opposite side of the substrate. This also results in increased capacitance, assuming that the corresponding conductive surfaces are not at the same electrical potential.
In another preferred embodiment, a sticker with a conductive layer is applied in the area within the coil so as to reduce the effective inductance of the resonant circuit. This influences the magnetic field inside the coil so that inductance is reduced, which produces a desired frequency increase. This measure is particularly effective if the sticker is positioned away from the center of the coil since the magnetic flux is denser here than in the center.
Instead of, or in addition to, a sticker, a local layer consisting of an initially liquid or pastelike and later drying or hardening material with a high dielectric constant can be used as an additional element according to the invention, with the local layer of this material being placed in the gap between two conductive surfaces with a different electrical potential. A layer of this type creates an additional dielectric between the conductors, which results in increased capacitance and hence a decrease in the frequency of the resonant circuit. The paste is easily applied between the two conductors. The dielectric of the paste can also be adjusted depending on the desired frequency adjustment.
Other embodiments follow from the dependent claims and from the following description of examples of embodiment.